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\documentclass[12pt]{article} % Chris Sangwin 2004-06-23
\usepackage{pstricks}
\usepackage{pst-plot}
\usepackage{multido}
\usepackage[nomessages]{fp}
\pagestyle{empty}
\parindent=0pt
%
\newwrite\DataFileA
\newwrite\DataFileB
%
\newcommand{\DataFileNameA}{MyCurveA.dat}
\newcommand{\DataFileNameB}{MyCurveB.dat}
%
\def\Xpsz{6}
\def\Ypsz{6}
\newcommand{\PerCalc}[2]{%
\def\ix{#1}\def\iy{#2}%
\FPeval{\iY}{\Ypsz * \iy / (\iy + 1.0)} % Calculates Y-coordinate
\FPeval{\iX}{(\Ypsz - \iY)* \ix / \Xpsz} % Calculates X-coordinate
}% End perspective calculate
\begin{document}
\begin{pspicture}(-6,0)(6,6)
\multido{\I=-6+1}{13}{\psline(0,6)(\I,0)}
% The horizontal perspective grid
\multido{\IR=0+1}{13}{%
\PerCalc{-6.0}{\IR}
\FPneg{\iXX}{\iX} % Exploits symmetry
\psline[linecolor=red](\iXX,\iY)(\iX,\iY)}% End of the horizontal grid
\immediate\openout\DataFileA=\DataFileNameA
\immediate\openout\DataFileB=\DataFileNameB
\multido{\IRx=0+1}{90}{%
\FPeval{\Ry}{ (\IRx * \IRx) / 30.0 } % Defines the quadratic
\PerCalc{\IRx}{\Ry}
\immediate\write\DataFileA{(\iX,\iY)}
\FPneg{\iXX}{\iX} % Exploits symmetry - so we'll have to loose this...
\immediate\write\DataFileB{(\iXX,\iY)}}
\immediate\closeout\DataFileA
\immediate\closeout\DataFileB
\readdata{\MyCurve}{\DataFileNameA}
\dataplot[plotstyle=curve,%
showpoints=true,%
linecolor=green,linewidth=1.5pt]{\MyCurve}
\readdata{\MyCurve}{\DataFileNameB}
\dataplot[plotstyle=curve,%
showpoints=true,%
linecolor=blue,linewidth=1.5pt]{\MyCurve}
\end{pspicture}
\end{document}
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